UNIVERSITY    OF    CALIFORNIA      AGRICULTURAL  EXPERIMENT  STATION 
COLLEGE    OF    AGRICULTURE  "NJ    'DE  WHtEL"'  >«••«-« 

THOMAS    FORSYTH     HUNT,    Dean    and    Director 
BERKELEY  H.    K.    VAN    NORMAN,    Vice-Director    and     oean 

University    Farm  School 


CIRCULAR  No.  171 
September,  1917 

THE   FERTILIZATION  OF  CITRUS 

By  W.  P.  KELLEY 


The  fertilizers  that  are  applied  to  the  citrus  groves  of  California 
differ  widely.  Many  growers  use  one  or  more  of  the  well  known 
brands  of  commercial  fertilizer;  others  apply  large  amounts  of  dried 
blood,  tankage,  bone  meal,  or  ammonium  sulfate ;  while  still  others 
rely  mainly  on  animal  manures,  bean  straw,  and  covercrops.  Fre- 
quently, fertilizers  of  widely  different  composition  are  applied  to 
adjacent  orchards  of  practically  the  same  age,  and  on  the  same  type 
of  soil.  Certain  fertilizer  materials  are  now  extremely  difficult  to 
obtain,  and  practically  all  of  them  that  are  of  special  importance  to 
California  orchardists  have  markedly  advanced  in  price  during  the 
past  three  years. 

In  the  use  of  fertilizers,  California  growers  were  formerly  com- 
pelled to  rely  mainly  on  the  experience  of  humid  sections.  More 
recently,  however,  various  investigations  have  been  made  on  this 
subject  in  California,  the  results  of  which  supply  information  of 
special  interest  at  the  present  time.  In  this  circular,  it  is  intended 
to  point  out  the  more  important  lessons  taught  by  these  investigations. 
It  should  be  recognized,  however,  that  a  wide  range  of  soil  conditions 
are  met  with  in  different  citrus  orchards,  and  that  the  fertilizer  needs 
vary  considerably.  Consequently,  it  is  not  possible  at  the  present 
time  to  make  definite  recommendations  that  will  apply  with  equal 
force  to  every  citrus  orchard.  It  is  hoped,  however,  that  the  indi- 
vidual citrus  grower  may  be  able  to  apply  the  suggestions  offered 
below  in  such  way  as  will  enable  him  to  better  meet  the  present 
fertilizer  situation,  and  at  the  same  time  supply  the  fertilizer  needs 
of  his  soil. 

ORGANIC    MATTER 

The  most  important  consideration  in  the  fertilization  of  citrus  in 
California  relates  to  the  organic  matter  of  the  soil.     The  virgin  soils 


are  commonly  low  in  organic  matter  and  the  tendency  is  ever  toward 
the  loss  of  this  constituent.  The  forces  which  bring  about  decompo- 
sition of  soil  organic  matter  are  stimulated  by  the  more  favorable 
conditions  which  ensue  as  a  result  of  cultivation  and  irrigation;  the 
temperature  and  moisture  conditions  are  suitable  for  rapid  decompo- 
sition throughout  the  main  part  of  the  year.  Consequently,  it  is 
necessary  to  put  forth  special  effort,  if  the  organic  matter  be  main- 
tained. 

Suitable  physical  conditions  in  soils  are  of  great  importance,  with- 
out which  no  amount  of  fertilizer  or  other  soil  treatment  can  be 
expected  to  produce  maximum  effects.  But,  the  maintenance  of 
suitable  physical  conditions  in  a  soil  is  more  dependent  on  the  pres- 
ence of  decaying  organic  matter  than  on  any  other  factor.  Likewise 
favorable  chemical  and  active  biological  conditions  in  soils  are  vitally 
dependent  on  the  presence  of  organic  matter.  In  fact,  organic  matter 
is  the  life  of  a  soil. 

As  a  means  of  supplying  organic  matter,  leguminous  covercrops 
are  recommended.  It  is  well  known  that  legumes  have  the  power 
of  fixing  nitrogen  from  the  air  and  thus  adding  it  to  the  soil,  but 
it  is  doubtful  whether  the  value  of  a  covercrop  can  be  adequately 
estimated  by  its  nitrogen  content.  The  effects  of  decaying  organic 
matter  on  the  soil  itself  are  probably  of  as  much  importance  as  the 
actual  nitrogen  added  from  the  air.  Leguminous  covercrops  undergo 
rapid  decomposition,  leaving  the  soil  in  a  physical  and  chemical  con- 
dition that  is  better  suited  to  plant  life.  In  order  to  avoid  bad  soil 
conditions  that  may  otherwise  arise,  it  is,  recommended  that,  wherever 
possible,  the  covercrop  be  incorporated  with  the  soil  not  later  than 
the  middle  of  March. 

A  second  important  means  of  replenishing  the  organic  supply  of 
soils  is  by  the  use  of  manure,  and  almost  every  orchardist,  whether 
he  be  a  citrus,  walnut,  or  deciduous  fruit  grower,  should  put  forth 
increased  effort  to  enlarge  and  conserve  his  supply  of  manure. 

In  general  the  greatest  value  to  the  soil  will  result  from  plowing 
down  the  manure  as  soon  as  possible  after  it  is  applied,  but  no  hard 
and  fast  rules  can  be  laid  down  regarding  the  time  of  application  or 
the  amount  to  be  applied.  As  a  general  rule,  better  soil  conditions 
result  from  making  the  application  early  than  late  in  the  season,  and 
the  amount  available  together  with  its  cost  will  rarely  permit  the 
grower  to  use  an  excess.  Ten  cubic  feet  per  tree  may  be  considered 
to  be  a  reasonable  application. 

Manures  differ  widely  both  in  composition  and  in  their  effect  on 
soils.  The  rates  of  decomposition  likewise  differ  widely.  Generally 
speaking,  the  more  straw,  sawdust,  shavings,  etc.,  a  manure  contains, 


the  more  slowly  it  will  undergo  decomposition,  and,  just  as  in  the 
case  of  covercrops,  manures  cannot  be  adequately  valuated  by  their 
plant  food  content.  The  nitrogen  content  of  manure  gives  to  it 
special  value  for  California  soils,  but  too  much  stress  should  not  be 
placed  on  the  analysis  of  manures,  for  it  is  beyond  doubt  that  the 
effects  produced  by  this  material  are  often  not  proportionate  to  the 
plant  food  content.  The  soils  need  to  be  enlivened  with  decajdng 
organic  matter,  and  manure  is  a  valuable  material  for  this  purpose. 
Another  important  source  of  organic  matter  is  found  in  bean 
straw  and  alfalfa  hay,  but  in  view  of  the  value  and  need  of  these 
materials  as  stock  feed  at  the  present  time,  the  citrus  growers  are 
advised  to  make  use  of  winter  covercrops  and  manures  instead. 
However,  bean  straw  and  alfalfa  hay  are  valuable  materials  for  use 
on  citrus  soils.  Their  high  nitrogen  content  makes  it  possible  to 
supply  a  liberal  amount  of  this  element  along  with  the  organic  matter, 
and  the  fact  that  these  materials  undergo  rapid  decomposition,  insures 
rapid  nitrification  and  therefore  prompt  availability. 

NITROGEN 

Among  the  common  elements  of  fertilizers,  nitrogen  is  by  far  the 
most  important  for  the  citrus  soils  of  California.  Most  of  these 
soils  are  notably  deficient  in  nitrogen;  the  citrus  tree  requires  rela- 
tively large  amounts  of  nitrogen  and  the  loss  of  nitrogen  through 
leaching  is  relatively  great.  All  parts  of  the  citrus  tree,  including 
the  wood,  leaves,  and  fruit,  contain  considerably  more  nitrogen  than 
any  other  fertilizer  element  and  experiments  in  different  localities 
show  that  marked  benefits  result  from  the  application  of  nitrogenous 
fertilizers.  In  fact,  all  of  the  various  lines  of  investigation  and  prac- 
tical experience  agree  in  giving  special  importance  to  the  use  of 
nitrogenous  fertilizers  on  the  citrus  soils  of  California. 

At  the  present  time,  it  appears  that  any  one  of  a  considerable 
range  of  nitrogenous  materials  may  be  used  with  fair  assurance  of 
success.  Dried  blood,  tankage,  whale  meat,  fish  scraps,  cotton  seed 
meal,  bone  meal,  the  nitrogen  -of  processed  fertilizers,  ammonium  sul- 
fate, etc.,  may  be  used  with  assurance  that  the  nitrogen  will  readily 
become  available.  Calcium  nitrate  may  also  be  expected  to  give  good 
results.  In  choosing  between  these  materials,  it  should  be  borne  in 
mind  that  their  nitrogen  content  varies  greatly,  and  that,  as  a  general 
rule,  their  value  as  fertilizers  for  citrus  in  California  is  proportionate 
to  the  nitrogen  content. 

The  amount  of  nitrogen  that  may  profitably  be  applied  will  depend 
on  at  least  three  factors:  First,  the  amount  of  growth  made  by  the 
leguminous  covercrop ;  second,  the  amount  of  manure  or  other  organic 


nitrogenous  material  that  is  applied;  and  third,  the  age  of  the  trees 
and  condition  of  the  soil.  In  general,  the  larger  the  trees  and  the 
heavier  the  crop,  the  more  nitrogen  is  required,  but  on  the  other  hand, 
the  better  the  growth  of  the  covercrop  and  the  heavier  the  application 
of  manure,  the  less  nitrogenous  fertilizer  will  be  needed.  The  grower 
can  best  be  guided  in  this  matter  by  the  experience  of  the  best  growers 
in  his  community,  together  with  his  own  local  conditions. 

The  nitrogen  of  nitrate  of  soda  is  also  readily  available,  and  in 
other  parts  of  the  world,  this  material  is  generally  considered  to  be 
one  of  the  most  valuable  sources  of  nitrogen.  It  has  even  been  used 
successfully  in  some  localities  in  California.  Nevertheless,  the  citrus 
growers  of  California  are  cautioned  against  its  use  here  for  the  reason 
that  in  many  cases  the  continued  use  of  nitrate  of  soda  has  resulted 
in  an  increased  tendency  toward  mottled  leaf.  In  view  of  this  fact, 
and  since  other  sources  of  nitrogen  are  available,  caution  is  advised 
in  the  use  of  this  material  on  citrus  soils. 

PHOSPHORIC   ACID 

The  second  most  important  fertilizer  constituent  for  the  citrus 
soils  of  California  is  phosphoric  acid,  but  the  need  is  much  less 
urgent  than  for  nitrogen.  The  requirements  of  the  citrus  tree  for 
phosphoric  acid  are  notable,  however,  and  man y  of  the  soils  contain 
comparatively  small  amounts  of  it,  consequently  it  will  probably  pay 
to  apply  limited  amounts  of  phosphoric  acid.  Superphosphate,  bone 
meal,  or  tankage  are  suitable  sources  of  this  constituent.  When  used 
in  intimate  contact  with  decaying  organic  matter,  rock  phosphate 
may  also  be  applied  as  a  source  of  phosphoric  acid. 

POTASH 

All  the  information  at  hand  points  to  the  conclusion  that  potash 
can  be  eliminated,  for  the  present  at  least,  from  the  citrus  fertilizers 
of  California  without  seriously  affecting  the  growth  of  the  trees  or 
fruit.  Many  of  the  soils  have  been  derived  from  granite  and,  gen- 
erally speaking,  the  soils  throughout  the  citrus  region  are  well 
supplied  with  potash.  In  addition,  the  effects  resulting  from  the 
application  of  potash  in  a  number  of  localities  have  been  negligible. 
Consequently,  it  is  fairly  safe  to  conclude  that  the  potash  already 
present  in  most  of  the  soils  will  become  available  sufficiently  fast  to 
meet  the  requirements  of  citrus  trees  for  a  number  of  years  at  least, 
without  any  artificial  application.  This  is  especially  true  where  the 
soil  is  kept  in  good  condition  through  the  use  of  covercrops  or  manure. 
In  view  of  these  facts,  and  since  the  amount  of  potash  available  for 
use  is  quite  limited,  the  use  of  potash  as  a  fertilizer  is  not  recom- 
mended at  the  present  time. 


